1. Field of the Invention
This invention relates to polyphase filters and, more specifically, to an eight-phase 45xc2x0 polyphase filter with amplitude matching.
2. Related Art
Certain radio receiver architectures require that signals be phase split equally into eight separate phases with constant 45xc2x0 phase splits between them. For example, direct conversion receivers and subharmonic frequency translators for use in such receivers may have such requirements. In addition, preprocessors for improving the switching characteristics of a local oscillator input to such frequency translators may also have these requirements.
One of the most common and most reliable ways of achieving phase splits in general is through the use of passive RC filters, that are commonly called polyphase filters. In the simplest RC series circuit, the voltage across the capacitor will lag the current, and the voltage across the resistor, by 90xc2x0. In a differential RC phase splitter, the two inputs are 180xc2x0 out of phase with each other and four outputs are available, each of which has a phase angle difference of 90xc2x0 relative to the next output.
By selecting the resistor and capacitor values, the phase angle of an output voltage relative to an input voltage can be achieved for a particular frequency. The inputs of two differential RC phase splitters can be connected. The resistor and capacitor values of the two phase splitters can be selected so that there is a 45xc2x0 phase angle difference between an output of the first phase splitter and a corresponding output of the second phase splitter at a particular frequency. The result is eight outputs with 45xc2x0 phase splits between them at a particular frequency.
The four outputs that are available from a differential RC phase splitter can be amplitude matched at a particular frequency by selection of appropriate resistor and capacitor values. However, the resistor and capacitor values cannot, in general, be selected to achieve both amplitude matching and a particular phase difference relative to the input.
A full eight-phase 45xc2x0 split is achieved by tying together the inputs of two offset four-phase 90xc2x0 phase splitters. The resistor and capacitor values are selected to obtain the 45xc2x0 phase angle difference between corresponding outputs of the two offset phase splitters. Amplitude matching is achieved by obtaining the inputs for that stage of the polyphase filter from an additional single four-phase 90xc2x0 phase splitter, whose resistor and capacitor values are selected to achieve amplitude matching. At a particular frequency, the additional phase splitter can distribute power evenly among the four inputs of the two offset phase splitters, so as to cancel out an uneven power distribution which otherwise would occur between the outputs of the two offset phase splitters.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.